Polyethylene terephthalate (PET) containers are normally produced by means of stretch blow moulding employing a hollow injection moulded preform.
The preform normally has a diameter which is less than the diameter of the container which is to be produced from the preform, an axial length which is less than the axial length of the container, and an opening which has the same dimensions and shape as the opening of the container.
The container is stretch blow moulded from the preform by firstly heating the preform to a temperature which will allow the preform to be moulded into the shape of the container. The heated preform is then positioned inside a mould having an interior shape corresponding to the exterior shape of the container which is to be moulded from the preform.
In conventional stretch blow moulding the pressure of the air inside the preform is increased to a “blowing pressure”, which is normally between 20 and 50 bar, so that the preform is expanded to such an extent that the exterior surfaces thereof press against the interior surface of the mould and thereby adopt the shape of the interior surface of the mould. Once the expanded preform has cooled sufficiently, the preform remains in the shape of the container without requiring the presence of the air at the blowing pressure.
The blowing pressure is normally generated by a pump in combination with a pressure vessel for storing the compressed air which is output by the pump. The pump and the pressure vessel must comply with certain regulations. Complying with these regulations usually involves a considerable investment of time and money.
FR 2839277 discloses a method of stretch blow moulding for producing a polyester resin container from a heated preform in which an incompressible fluid is injected in to the preform. In the process of FR 2839277 the injection of incompressible fluid simultaneously stretches and inflates the preform until it reaches the walls of the mold. The method of FR 2839277 is particularly suited to a single stage combined molding-filling embodiment for the production of small containers such as PET drink bottles.
Other stretch blow moulding methods use a compressible fluid, such as a gas, in combination with a stretching rod. In these methods the stretching rod is introduced axially into the mould and into the interior of the preform through the opening thereof, to stretch the heated preform inside the mould.
The stretch blow moulding processes described above are particularly suitable for the manufacture of relatively small plastic containers having volumes of up to approximately 20 liters and wall thickness of approximately 0.3 millimetres. However, for containers and articles having larger volumes and for wall thicknesses, the processes are uneconomic and unreliable as they are unable to reliably produce uniformly expanded containers and articles of about 20 liters volume or greater.